This invention relates to a method for forming a solder-bumped terminal on a printed circuit board or the like. More particularly, this invention relates to such method comprising reflow of a thin solder plate on a circuit trace to cause the solder to coalesce onto a terminal pad and form the desired bump.
In the manufacture of a microelectronic package, it is known to mount an integrated circuit component onto a printed circuit board by a plurality of solder bump interconnections. The board comprises a copper circuit trace disposed on a dielectric substrate and including terminal sections, each ending at a pad to which the solder interconnection is bonded. Typically, the pad has the same width as the adjacent trace and is demarked by a dam formed of a solder-nonwettable material, referred to as a solder stop, to confine the solder to the pad. The component, which may be, for example, an integrated circuit die attached to a ceramic carrier, has a plurality of solderable bond pads arranged in a pattern superposable on the terminal pads.
In preparation for mounting the component to the board, a solder bump is attached to each bond pad on the component, typically by placing a preformed microsphere of the solder on the pad and briefly heating to reflow the solder. The bumped component is assembled with the board such that each bump rests against a corresponding terminal pad on the board. The assembly is heated to concurrently reflow the several bumps onto the adjacent terminal pads to form the interconnections. During reflow, each bump is heated to melt the solder to wet the pad and resolidified, whereupon the solder becomes bonded to the terminal pad, as well as the bond pad on the component, to complete the interconnection. The interconnections not only physically join the component to the board, but also electrically connect the component to the circuit trace on the board for conducting electrical signals to and from the component for processing.
In the formation of solder bump interconnections, it is found to be advantageous to apply a solder bump to the terminal pad of the circuit trace in addition to the bump applied to the component. The solder bump may be formed on the circuit board by applying and reflowing a preformed microsphere in a manner similar to the component. However, it is more convenient to form the bumps on the board by electroplating, in part because the relatively large size of the board makes microsphere placement more cumbersome and also because the trace is available for distributing the necessary current to the several terminal pads for simultaneous plating. Processes are available for plating a thin layer of solder onto circuit traces to protect the copper surface against oxidation during manufacture of the board that would otherwise interfere with subsequent soldering of leads, for example, in the attachment of discrete components, such as resistors or capacitors. However, such thin films have not heretofore been employed in forming solder bump interconnections. This is in part because the thin film does not provide a sufficient mass for the solder bump, whereas a thick plate applied to the trace not only increases the amount of solder substantially and unnecessarily, particularly since the terminal pads constitute but a minor portion of the trace, but also because thick solder adjacent to the pad would interfere with the interconnection. Thus, a mask is needed to limit deposition to the terminal pads, which is in addition to the mask used to define the trace and which requires further processing steps to apply, develop, and remove the mask, increasing the cost of the package. In addition, the plating time needs to be extended to deposit the required thickness onto the limited area of the terminal pad. Thus, there remains a need for an improved method for forming bumps that benefits from the advantages of depositing the solder by electroplating, but does not require an additional mask and minimizes plating time while forming a bump of adequate mass for completing the solder bump interconnections.